The process of resistance welding involves positioning a workpiece to be welded between a pair of electrodes through which electrical current is delivered from a source thereof. The electrodes are clamped under pressure to squeeze the workpiece therebetween so that electrical current is delivered from one electrode through the workpiece to the other electrode at the point of contact between the electrodes and the workpiece. Heat generated by the resistance encountered by the current passing through the parts of the workpiece to be joined melts the contact faces of the parts, thereby melting the parts to create a weld. In some cases, after the welded joint between the parts has reached a sufficiently high temperature, electrical current flow is terminated and the clamping pressure is maintained for a prescribed time interval to unite the parts. In other cases, such as spot welding, the parts need not remain clamped after current flow is terminated.
One type of resistance welding apparatus is described in U.S. Pat. No. 3,074,009 in which the electrodes receive electrical current from the secondary windings of the transformer. The primary winding of the transformer is coupled in series with a capacitor, an electrical supply source and a pair of ignitrons or similar rectifier tubes. The ignitrons are phase shifted such that they begin to conduct just before the voltage peak in each half cycle. Each time an ignitron begins to conduct, it places a charge on the capacitor approximately equal to this peak. The charge remains on the capacitor until the next half cycle when the other ignitron fires. The capacitor rapidly discharges the stored potential into the primary transformer thereby generating a sharp pulse in the transformer secondary circuit which is delivered to the electrodes. Current is therefore delivered to the electrodes from the secondary transformer in a series of spikes or pulses each equal to approximately twice the value of the instantaneous line voltage. This pulsed current supply provides instantaneous localized heating of the welding surface which cannot be achieved by a lower current of proportionally longer duration because of the dissipation of the interface surface temperature by conduction. Thus, the lower value of power is required in these pulse current systems and undesirable secondary heating effects are minimized.
In order to adapt pulse power type welding apparatus for use in various applications, control systems have been devised which allow selective control of each sequence in a welding cycle, i.e., total clamping time, welding time, holding time after termination of welding current, as well as the magnitude of current delivered to the electrodes. These prior art systems employed mechanical timers and potentiometers arranged in an analog circuit to control the various process variables. Systems of this type were undesirable, however, in that the process parameters could not be selected and controlled with the repeatable precision required in many applications. For example, potentiometers and the like employed to select process parameters could not be precisely reset. Also, deterioration of tubes and other electrical components after a period of use adversely affected the overall accuracy of the system.
Accordingly, it is an important object of the present invention to provide a solid state control system employing a microcomputer for accurately and reliably controlling a resistance type pulse power welding apparatus.
A further object of the present invention is to provide welding apparatus and a control system therefor as described above which is highly flexible in terms of its adaptability to various welding applications.
A further object of the invention is to provide a control system as described above in which process parameters are precisely selected and stored in a memory for use in controlling the welding apparatus.
A still further object of the invention is to provide a control system as described above which includes a sensing circuit for detecting the level of line voltage and inhibiting the operation of the microcomputer when the line voltage falls below a prescribed level thereof.
These and further objects of the invention will be made clear, or will become apparent during the course of the following description of a preferred embodiment of the invention.